Various investigations have been conducted on a white light-emitting organic EL device because the device can be used in a lighting application such as a backlight or a full-color display device using a color filter. The white light-emitting organic EL device has a structure obtained by laminating multiple light-emitting layers, and realizes the emission of white light by mixing luminescent colors from the respective light-emitting layers. The chromaticity of light emitted from each light-emitting layer in such white light-emitting organic EL devices, and the efficiency with which the light-emitting layer emits the light are controlled by trial and error because the chromaticity and the efficiency each vary to a large extent depending on, for example, the amount of dopant in the light-emitting layer (doping ratio) and the thickness of the light-emitting layer.
In view of the foregoing, the following method has been proposed: in order that the emission of white light with a target chromaticity may be efficiently designed, the thickness and doping ratio of each light-emitting layer are each set by using luminous efficiency and the chromaticity coordinates of the color of emitted and extracted light as parameters, and the thickness of at least one of a hole-transporting layer and an electron-transporting layer is set by using the chromaticity coordinates of the color of the emitted and extracted light as parameters in association with the set thickness of each light-emitting layer. For example, JP 2004-63349 A discloses a method of designing a white light-emitting organic EL device in which three light-emitting layers are laminated.
With the related method described in JP 2004-63349 A, the chromaticity of the device can be brought close to a target one in a somewhat efficient fashion by using a large number of parameters including the thickness of each light-emitting layer, the doping ratio of each light-emitting layer, the thickness of the hole-transporting layer, and the thickness of the electron-transporting layer. In actuality, however, the method involves the following problems: the chromaticity coordinates of the device cannot be easily adjusted to target ones with accuracy because the number of parameters to be adjusted is so large that it is difficult to combine the parameters with accuracy and because it is difficult to select, for example, a material for use in each of the light-emitting layers and the order in which the light-emitting layers are laminated appropriately. Also, investigations on the material and the order are still insufficient.